In this post, our Science Team explain 6 difficult concepts you must know for the HSC.
There are 6 difficult HSC Biology concepts students struggle with in the HSC Biology exam. Often this is caused by confusing two similar concepts. Make sure you have the following 6 HSC Biology concepts clarified!
Students often get amino acids, polypeptides and proteins confused. It is important to understand these terms in order to understand translation and the experiments of Beadle and Tatum.
Amino acids form the building blocks of the proteins in our body. They are composed of different arrangements of Carbon Hydrogen Oxygen and Nitrogen (CHON). Some amino acids are absorbed from our food (essential) and others need to be synthesised (non-essential).
Recall that amino acids are joined together by peptide bonds during the translation process to form polypeptides.
Individual amino acids are joined together to form polypeptides. Proteins are composed of one or more polypeptides that have been joined together and given a 3D configuration. Until they have the correct configuration, proteins will not be ‘functional’.
Students can get these confused as they both involve unzipping of the DNA in the first stage, but the end products are very different.
DNA replication only occurs in preparation for cell division (meiosis or mitosis). The DNA is unzipped by helicase, then each half of the DNA strand is used as a template. Complementary nucleotides are added by DNA polymerase and the end product is two identical copies of a chromosome.
Transcription and Translation
Transcription and translation (protein synthesis) happens during interphase. Inside the nucleus, DNA is unzipped by helicase, but only at the location of a specific gene. A single strand of complementary mRNA is produced by RNA polymerase and then the mRNA leaves the nucleus. Transcription is complete.
In the cytoplasm a ribosome ‘reads’ the mRNA. tRNA molecules enter the ribosome and drop-off amino acids. The ribosome joins the amino acids together using peptide bonds, the end product is a polypeptide chain.
Transcription and translation
|Only before cell division||During interphase|
|Two identical copies of a chromosome||A polypeptide|
Units of end product
|In the nucleus||Transcription in the nucleus then translation in the cytoplasm|
Remember: all enzymes are proteins, but not all proteins are enzymes!
In order to understand the experiments conducted by Beadle and Tatum, students must recall that enzymes can catalyse reactions to change one product into another. In this way, they can be used to synthesise new amino acids from existing ones.
Beadle and Tatum’s experiment used a bread mould that needs the amino acid Arginine to grow. If Arginine isn’t available in the immediate environment the bread mould can synthesise Arginine from the amino acid Glutamate using four different enzymes. This is referred to as the Arginine synthesis pathway.
At the time, the Arginine synthesis pathway was well understood, but the role of DNA in this pathway was unknown. Beadle and Tatum had a hypothesis that active DNA segments called genes coded for each enzyme.
To test this hypothesis they exposed many samples of the bread mould to radiation in order to cause mutations of its DNA. Different samples of bread mould ended up with different mutations.
In some samples the bread mould didn’t grow as the mutations had interrupted the Arginine synthesis pathway. For example, in some samples it was found that the Arginine synthesis pathway was stopping at Citruline. However, when the bread mould was supplied with Arginosuccinate in the growth medium it could synthesise Arginine and grow normally.
This indicated to Beadle and Tatum that the bread mould could synthesise Citruline, but not Arginosuccinate (and therefore not Arginine). The radiation had caused a mutation on the gene for Enzyme 3 on the Arginine synthesis pathway.
By producing strains of bread mould with different mutations, Beadle and Tatum could repeat the process to identify the gene for each of the four enzymes.
The one gene, one enzyme (or protein) hypothesis was later changed to the one gene, one polypeptide hypothesis when it was discovered that each gene codes for an individual polypeptide and that multiple genes can be involved in coding for an enzyme (or protein).
Whole organism cloning typically involves taking an unfertilised egg cell from one individual and removing the nucleus. A nucleus is then taken from a somatic cell (body cell) of a different individual and inserted into the egg cell. The egg is stimulated (to mimic fertilisation) and the egg cell should start to divide and form an embryo.
The embryo is then implanted in a surrogate mother. The resulting offspring will only have physical characteristics of the individual that donated the nucleus, as this is its only source of DNA. The typical example of this is Dolly the sheep.
In gene cloning a gene of interest is identified, often on the human genome, and cut out using restriction enzyme. The DNA (plasmid) of a bacterium is then cut open (using restriction enzyme again) and the gene is inserted into the plasmid using DNA ligase. The bacteria is then encouraged to multiply resulting in many copies of the gene of interest.
The gene may be multiplied so it can be inserted into a different organism later (e.g. a plant), or the gene may be expressed by the bacteria (via transcription and translation) so that it produces a useful product (e.g. insulin). Some students forget to include the role of plasmids and bacteria when describing gene cloning.
Antigens are proteins found on the surface of pathogens such as bacteria or viruses that enter the body.
Antibodies are produced by B-Cells and bind to the antigens helping to disable the pathogen and mark it for destruction.
Antibodies are produced by your body, antigens (antibody generators) are from outside of the body!
Unfortunately antigens are found on some substances that are not actually pathogens such as pollen or transplanted organs.
Your organs have antigens on their surface. Your immune system recognises these as part of your body and doesn’t launch an immune response against them.
However, organs or parts of organs can be transplanted from the body of a donor into the body of a patient.
The donated organ will have surface antigens unique to the donor and so the patient’s body recognises these as foreign. The immune system will attack these foreign cells and try to destroy the donated organ.
The patient is given immune suppressants to stop the body from attacking the donated organ, but this leaves the patient more susceptible to actual pathogens that cause illness. The patient will have to remain on immune suppressants for the rest of their life.